Method for reducing hydrogen sulfide emissions from wastewater

ABSTRACT

A cost-effective method for reducing the dissolved sulfide content in a wastewater stream and thereby hydrogen sulfide emissions therefrom involving the steps of adding a transition metal salt to the wastewater stream at the upper reaches of a wastewater collection system prior to at least some hydrogen sulfide volatilization followed by addition of an oxidant to the wastewater stream to generate elemental sulfur and a transition metal salt which subsequently participates in additional hydrogen sulfide capturing steps, thereby also improving water quality and wastewater treatment plant operations.

BACKGROUND OF THE INVENTION

The present invention relates to a method for reducing the dissolvedsulfide content within a wastewater stream and thereby the hydrogensulfide emissions there from and for improving water quality andtreatment plant operations by employing a transition metal salt and anoxidant as additives to the wastewater stream.

Hydrogen sulfide (H₂S) is a toxic, corrosive gas that is generatedwithin the biomass adhered to pipe walls and sediment of a sewagesystem. As wastewater is conveyed through the sewage collection systemto the wastewater treatment plant, septic conditions develop that fosterthe growth of hydrogen sulfide-producing bacteria. Hydrogen sulfidevolatilizes from the wastewater into the vapor space of the sewagesystem where it creates the problems of nuisance odors, infrastructurecorrosion, and worker hazards.

The use of iron salts alone to control hydrogen sulfide emissions inwastewater is known in the industry. Iron salts control hydrogen sulfide(H₂S) by converting volatile H₂S dissolved in the wastewater intononvolatile iron-complexed sulfide (FeS):

H₂S+FeCl₂→FeS+2 HCl.

Ferrous sulfide (FeS) is a black precipitate that is stable in theabsence of acid and typically settles out in clarifiers at thewastewater treatment plant, where it enters the solids stream. Thestoichiometric chemical requirement is 1.7 pounds Fe (or 3.7 poundsFeCl₂) per pound H₂S controlled, yielding a cost of approximately $0.50per pound hydrogen sulfide depending on the per-unit chemical cost.Additionally the efficiency of iron salts is not impacted by oxygenuptake rates within the wastewater.

Despite these advantages, the use of iron salts alone to control H₂Semissions in wastewater has shortcomings. Iron salts alone loseefficiency when achieving H₂S emissions control for more than about fourhours hydraulic retention time. Hydraulic retention time is defined asthe length of time a component resides within the sewage system. Thusthe efficient use of iron salts alone to control H₂S emissions ofwastewater requires a series of iron salt injection facilities locatedalong the course of the wastewater collection system. At each injectionsite, the spent iron salt (FeS) is augmented with fresh iron salt. Asused herein, the term “iron salt” refers to nearly any iron compound (asdistinguished from elemental iron) and expressly includes iron hydroxide(Fe(OH)₂, Fe(OH)₃, FeCl₃, FeCl₂, FeSO₄, and Fe₂(SO₄)₃) but excepts FeS,which is often referred to herein as “spent iron salt.” The spent ironsalt largely remains inert throughout the treatment and disposalprocesses. When the wastewater reaches the treatment plant, the mass ofspent iron salt settles out in the primary clarifiers. The iron saltdemand increases 2-4 fold to achieve H₂S emissions control for greaterthan about four hours hydraulic retention time, thus increasing theamount of spent iron salt generated. The FeS precipitates andconstitutes a theoretical solids load of about 3 pounds per pound H₂Scontrolled. The FeS precipitate can cause deposition problems within thesewage system, particularly in low-velocity sewage systems andclarifiers/thickeners as it settles out, thus increasing the actual costper pound H₂S controlled by 20% ($0.075) or more.

Iron salts also degrade the quality of wastewater. The salinity ofwastewater is increased by the addition of iron salts, as a minimum of 3pounds sodium chloride per pound H₂S controlled is generated when FeCl₂or FeCl₃ is used as the iron salt. Iron salts also deplete thealkalinity of the wastewater stream by consuming a minimum of 3 poundscalcium carbonate per pound H₂S controlled. Further, iron salt productstypically contain 1-4% mineral acid that further depresses the pH of thewastewater. The reduced alkalinity of the wastewater stream in turnreduces the capturing capacity of iron, thus reducing its ability tocontrol H₂S to low levels. Furthermore, the depressed pH of thewastewater encourages volatilization of untreated H₂S within thewastewater stream. Additionally, iron salts deplete the wastewaterstream of dissolved oxygen by consuming a minimum of 5 pounds dissolvedoxygen per pound H₂S controlled. Thus, while iron salts are useful incontrolling H₂S emissions of wastewater, it is desirable to minimize theamount of iron salt added to the wastewater stream to minimize thedisadvantages associated with the use of iron salts.

It has been reported that a blend of 1 part ferrous to 2 parts ferriciron provides improved control of H₂S emissions from a wastewater streamwhen compared to either ferrous or ferric iron alone. Such a blend,however, is expensive and is subject to the same disadvantages of ironsalts previously stated.

The use of hydrogen peroxide (H₂O₂) alone to control H₂S emissions isalso conventional. Like iron salts, H₂O₂ injection facilities within thesewage system are typically located in series, separated by 1-2 hourshydraulic retention time. The use of hydrogen peroxide alone controlsH₂S emissions in wastewater by two mechanisms: direct oxidation of H₂Sto elemental sulfur (I) or prevention of H₂S formation by supplyingdissolved oxygen (II):

H₂S+H₂O₂→S_(o)+2H₂O  (I)

2H₂O₂→O₂+2H₂O  (II).

Direct oxidation theoretically requires 1.0 pound H₂O₂ per pound H₂Scontrolled at a cost of about $0.50 per pound H₂S and generates 1.0pound solids per pound H₂S controlled, regardless of H₂O₂ dose. Incontrast, prevention of H₂S formation by providing a dissolved oxygensupply theoretically requires 4.0 pounds H₂O₂ per pound H₂S controlledat a cost of $2.00 per pound sulfide. The second mechanism also isadversely affected by environmental factors such as hydraulic retentiontime and oxygen uptake. Thus, the practical H₂O₂ requirement can be 2-4times the theoretical H₂O₂ requirement when retention time increases by2-3 hours. Therefore, previous H₂O₂ applications within the municipalwastewater treatment industry are either targeted at point source H₂Scontrol, such as at the headworks to treatment plants, where H₂O₂ may beapplied to the wastewater stream to maximize its most efficient mode asan oxidant, or added as a preventative within the wastewater collectionsystem, at costs exceeding $2.00 per pound H₂S controlled.

While the independent use of H₂O₂ to control H₂S emissions by wastewatergenerates no adverse by-products and advantageously oxygenates thewastewater, it presents several shortcomings. Specifically, theoxidation reaction typically requires 15-30 minutes. In addition,control of H₂S emissions at 1-2 hours hydraulic retention time or moreis expensive, requiring double the injection stations required by FeCl₂control. Furthermore, the efficiency of H₂O₂ is adversely affected byhigh oxygen uptake rates. Hence, a mechanism which provides greater andmore efficient H₂S emissions control within the wastewater treatmentsystem is desirable.

A process for the conversion of aqueous hydrogen sulfide in geothermalsteam employing hydrogen peroxide and iron compounds is taught by U.S.Pat. No. 4,363,215 to Sharp. In the disclosed process, hydrogen sulfideis reacted with hydrogen peroxide, wherein the iron compound serves as acatalyst to accelerate the reaction of hydrogen peroxide with hydrogensulfide. The iron compound catalyst is added in an amount of from 0.5 to1.0 parts per million expressed as free metal and thus does not complexwith the sulfide. U.S. Pat. No. 4,292,293 to Johnson et al. furtherdiscloses the addition of polyanionic dispersants to improve theefficiency of the metallic ion catalyst for the oxidation of sulfide byhydrogen peroxide.

The addition of a combination of a ferric salt and an anionic polymer toa water clarifier is a known development in enhancing solids separationand thereby improving the cost-performance of wastewater treatmentplants, though such treatment has not yet been widely employed withinthe industry.

It is the object of the present invention to provide a cost-effectivemeans for reducing hydrogen sulfide emissions throughout the wastewatercollection system as well as the wastewater treatment plant whileimproving water quality and treatment plant operations.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a novel method for integrating the use oftransition metal salts and oxidants, specifically iron salts andhydrogen peroxide, respectively, to achieve reduced dissolved sulfidelevels and thereby H₂S levels within a sewage system. Iron salts areadded to the wastewater stream at the upper reaches of the wastewatercollection system prior to H₂S volatilization to capture H₂S dissolvedin the wastewater. The captured sulfide, as ferrous sulfide, is thendelivered to one or more points downstream of the iron salt additionwhere hydrogen peroxide is added to the wastewater stream. The hydrogenperoxide destroys the ferrous sulfide and restores the sulfide-capturingcapacity of the iron. At the final regeneration point, for example thewastewater treatment plant, the restored iron salt is used to enhancesolids separation and sulfide control in primary clarifiers as well assulfide and struvite control in anaerobic digesters.

DETAILED DESCRIPTION OF THE INVENTION

Applicants have discovered a novel process for controlling hydrogensulfide emissions from wastewater and for improving water quality andwastewater treatment plant operations in a cost-effective manner. Themethod of the present invention involves the use of transition metalsalts to capture sulfides generated within a sewage system and todeliver the captured sulfides to an oxidant added at a point downstreamof the transition metal salt addition. The oxidant restores thecapturing capacity of the transition metal by regenerating a transitionmetal salt from the spent transition metal salt (transition metalsulfide), thus allowing the regenerated transition metal salt toparticipate in the capture of additional hydrogen sulfide molecules. Thetransition metal salt is preferably a ferrous or ferric salt in asolution or any readily water-soluble form. For example, the iron saltmay be ferrous or ferric sulfate, chloride, nitrate, bromide, bromate,or a mixture thereof. The oxidant is preferably hydrogen peroxide.

The transition metal salt, preferably iron salt, is added to awastewater stream at the upper reaches of a wastewater collection systemprior to hydrogen sulfide volatilization. While hydrogen sulfide willbegin volatilizing almost immediately, the term “prior to hydrogensulfide volatilization” is intended to mean prior to somevolatilization, not necessarily prior to all volatilization. The ironsalt will aid in preventing future H₂S volatilization regardless ofwhether H₂S has volatilized previously. It would be practicallyimpossible to introduce iron salt prior to all H₂S volatilization. Thegreater the amount of hydrogen sulfide already present in the wastewaterstream at the point of iron salt addition, the greater the benefit ofadding ferric salt to control hydrogen sulfide emissions, instead offerrous salt, as ferric salt has an oxidizing capacity, albeit small. Anoxidant, preferably hydrogen peroxide, is then added to the wastewaterstream at one or more points downstream of the iron salt addition, toregenerate iron salt in situ. The hydrogen peroxide oxidizes the ironsulfide formed, to produce ferric hydroxide and/or ferrous hydroxidesalts.

Multiple regeneration steps using a series of hydrogen peroxideadditions spaced at points separated by approximately 4 hours hydraulicretention time may be used where the water collection system is long. Inaddition, hydrogen peroxide is preferably added to the influentwastewater stream at the wastewater treatment plant, the finalregeneration point. The iron hydroxide produced enhances solidsseparation and sulfide control in primary clarifiers, as well as sulfideand struvite control in anaerobic digesters at the treatment plant. Ananionic polyelectrolyte may be added to the influent of a primaryclarifier at the wastewater treatment plant to further improve solidsseparation.

The present invention may be represented as the following catalyticcycle, where a working inventory of iron is maintained with hydrogensulfide (H₂S) as the input, elemental sulfur (S_(o)) as the output, andhydrogen peroxide (H₂O₂) as the driver:

The preferred embodiment of the process occurs in three steps: (1) ironcomplexation with dissolved sulfide; (2) direct H₂O₂ oxidation of theFeS complex to provide elemental sulfur and ferric hydroxide (Fe(OH)₃);and (3) oxidation of additional sulfide by the ferric hydroxide toproduce elemental sulfur and FeS. The second and third steps are thenrepeated as additional hydrogen peroxide is used to regenerate ferrichydroxide from the ferrous sulfide. The preferred process may beexemplified in the following overall reaction:

Step 1: 2 H₂S + 2 FeCl₂ → 2 FeS + 4 HCl Step 2: 2 FeS + 3 H₂O₂ → 2S_(o) + 2 Fe(OH)₃ Step 3: 2 Fe(OH)₃ + 3H₂S → S_(o) + 2 FeS + 6 H₂O Netreaction: 5 H₂S + 2 FeCl₂ + 3H₂o₂ → 3 S_(O) + 2 FeS + 6 H₂O + 4 HCl.

This net reaction stoichiometrically requires 0.67 lbs Fe (or 1.45 lbsFeCl₂) and 0.6 lbs H₂O₂ per lb sulfide, to yield a theoretical cost ofabout $0.50 per lb sulfide controlled. This is based on 0.67 lbs Fe²⁺.Commensurately less iron would be required if introduced as Fe³⁺. Inthat case, only 0.5 lbs Fe³⁺ and 0.6 lbs H₂O₂ per lb sulfide controlledwould be needed.

The method of the present invention achieves a number of advantages overconventional methods. In sharp contrast to conventional treatmenttechniques the addition of fresh iron salt downstream of the initialinjection site or even at the treatment plant is not required, as amixture of ferric and ferrous salts is provided by in situ regenerationof spent iron salt by hydrogen peroxide in the wastewater collectionsystem and upon entry of the treatment plant. Because iron salt needonly be added to the wastewater stream at one point in the collectionprocess and is regenerated thereafter by hydrogen peroxide, the presentinvention requires only a fraction of the iron input required by therelated art. Thus, the present invention achieves a greater than 40%reduction in solids production, a greater than 60% reduction in aciditycontribution, and a greater than 80% reduction in dissolved oxygendemand. These benefits result in an overall cost savings by reducing thesolids load and the amount of iron salt required, thus reducing thesolids generated and associated disposal cost.

Additionally, the iron levels in the wastewater stream augmented byreaction of FeS with H₂O₂ increases the removal rate of H₂S by more than90%, thus significantly improving the degree of H₂S control afforded ascompared to the use of iron salt or H₂O₂ alone. For example, the use ofH₂O₂ alone to control sulfide emissions requires H₂O₂ addition at a site20-40 minutes hydraulic retention time upstream of the point of H₂Srelease. In sharp contrast, the present invention allows the addition ofH₂O₂ to be located at or 1-10 minutes prior to the point of desired H₂Scontrol.

Regeneration of iron salt with H₂O₂ also results in a mixture of ferricand ferrous salts having superior H₂S capturing capacity as compared toferric or ferrous salt alone. The ferrous-to-ferric iron ratio of thisblended product may be adjusted by varying the H₂O₂ dose.

The present invention represents a novel and significant improvement forreducing H₂S emissions from wastewater. In addition to providingpractical, long-duration H₂S control to low sulfide levels via a rapidoxidative reaction, the method of the present invention providessignificant treatment plant benefits. The invention being thusdescribed, it will be obvious that the same may be varied in many ways.Such variations are not to be regarded as a departure from the spiritand scope of the invention, and all such modifications as would beobvious to one skilled in the art are intended to be included within thescope of the following claims.

We claim:
 1. A method for reducing the evolution of hydrogen sulfidevapors within a sanitary sewer system, comprising the steps of: (a)adding an iron salt to a wastewater stream within said sanitary sewersystem upstream of hydrogen sulfide volatilization to produce free ironions which react with said hydrogen sulfide to form iron (II) sulfide;and (b) adding deliberately an oxidant to said wastewater streamdownstream of said iron salt addition to regenerate free iron ions fromsaid iron (II) sulfide wherein, in said method, no nitrate ion is addedto said wastewater stream.
 2. The method of claim 1 wherein said oxidantis hydrogen peroxide.
 3. The method of claim 1 wherein said iron salt isselected from the group consisting of ferrous chloride, ferrous sulfate,ferric chloride, ferric sulfate, and mixtures thereof.
 4. The method ofclaim 1 wherein said regenerated free iron ions are ferric ions.
 5. Themethod according to claim 1, further comprising the step of: (c) addingdeliberately an oxidant to said wastewater stream downstream of saidoxidant addition step (b), wherein the oxidant of step (c) may be thesame oxidant as the oxidant of step (b) or may be a different oxidantthan the oxidant of step (b).
 6. The method of claim 5, wherein saidoxidant addition of step (c) occurs at or upstream of a wastewatertreatment plant.
 7. The method of claim 1, wherein said oxidant is addedto said wastewater stream in a stoichiometric amount of oxidant perpound sulfide controlled.
 8. The method of claim 2, wherein saidhydrogen peroxide is added to said wastewater stream in a stoichiometricamount of oxidant per pound sulfide controlled.
 9. The method of claim 2wherein said hydrogen peroxide is added to said wastewater stream in anamount of at least 1.0 lbs H₂O₂ per pound sulfide controlled.
 10. Amethod for reducing the evolution of hydrogen sulfide vapors within asanitary sewer system, comprising the steps of: (a) adding an iron saltto a wastewater stream within said sanitary sewer system upstream ofhydrogen sulfide volatilization to produce free iron ions which reactwith said hydrogen sulfide to form iron (II) sulfide; (b) addingdeliberately an oxidant to said wastewater stream downstream of saidiron salt addition to regenerate free iron ions from said iron (II)sulfide; and (c) adding an anionic polyelectrolyte to said wastewaterstream at said wastewater treatment plant.
 11. A method of enhancingsolids separation in a primary clarifier comprising: (a) adding an ironsalt to a wastewater stream in a wastewater collection system upstreamof hydrogen sulfide volatilization to produce free iron ions which reactwith said hydrogen sulfide to form iron (II) sulfide; (b) addingdeliberately an oxidant to said wastewater stream downstream of saidiron salt addition to regenerate free iron ions from said iron (II)sulfide, which free iron ions react with said hydrogen sulfide to reformiron (II) sulfide; and (c) adding deliberately an oxidant to saidwastewater stream at the inlet of a wastewater treatment plant prior toentry of said wastewater to said primary clarifier.
 12. The method ofclaim 11 wherein said oxidant is hydrogen peroxide.
 13. A method oftreating wastewater at a wastewater treatment plant comprising: (a)adding an iron salt to a wastewater stream in a wastewater collectionsystem upstream of hydrogen sulfide volatilization to produce free ironions which react with said hydrogen sulfide to form iron (II) sulfide;(b) adding deliberately an oxidant to said wastewater stream downstreamof said iron salt addition to regenerate free iron ions from said iron(II) sulfide, which free iron ions react with said hydrogen sulfide toreform iron (II) sulfide; and (c) adding deliberately an oxidant to saidwastewater stream at the inlet of a wastewater treatment plant toregenerate free iron ions from said reformed iron (II) sulfide.
 14. Themethod of claim 13 wherein said oxidant is hydrogen peroxide.